1. Field of the Invention
The present invention relates to an optical recording medium recorded with a hologram and an apparatus of judging a fake, particularly, relates to an optical recording medium suitable for recording all of or a part of information by a hologram on a medium such as a pre-paid card, a cash card for a cash dispenser or an automatic teller machine, a driver license or a membership card, and an apparatus of judging a fake of such a card or information.
2. Description of the Related Art
FIG. 1 shows a binary optics reading apparatus disclosed in the Japanese Patent Laid-open Publication No. 9-319849/1997 as the prior art. Binary optics 10a through 10c, the reference number of the binary optics is hereinafter called as 10 in general, formed on a card 1 are diffraction gratings, which are designed for concentrating intensity of diffraction light on the first order and the second order diffraction light 23a and 23b, when they are irradiated by monochromatic light 21 emitted from an illuminating device 20 through a half mirror 25. These first order diffraction light 23a and second order diffraction light 23b are received by a first order diffraction light receptor 30a and a second order diffraction light receptor 30b respectively. Light reflected by the half mirror 25 is received by an illuminating light receptor 24 through the half mirror 25 so as to detect intensity of the illuminating light or the monochromatic light 21. A diffraction efficiency of the diffraction grating 10 can be obtained by dividing a sum of an intensity of the first order diffraction light 23a and an intensity of the second order diffraction light 23b by an intensity of the illuminating light or the monochromic light 21. Therefore, it is judged that the card 1 is a "true" card, in a case that the diffraction efficiency exceeds a predetermined value and it is judged that the card 1 is a "fake" card, in a case that the diffraction efficiency is less than the predetermined value. Accordingly, the prior art standardizes diffraction efficiency for judging the validity of the card 1 as "true" or "fake".
The diffraction grating 10 utilized in the prior art is a phase hologram, which is described in the "O plus E" magazine, Dec. 1996 issue, on pages 83 through 88 titled as "The most suitable method of computer generated hologram". Specifically, phase distribution in response to interference fringes of a diffraction grating can be obtained by the Fourier reverse transform of location information of diffraction light in response to a desired angle of diffraction light by a computer. The diffraction grating 10 can be formed on an optical recording medium such as a card by transferring a blaze in response to the phase distribution formed on a substrate, wherein the blaze is formed on a substrate by applying such the semiconductor manufacturing technology disclosed in the Japanese Patent Laid-open Publication No. 9-230121/1997. Such a diffraction grating element manufactured by the process mentioned above is called a binary optics.
A phase hologram is explained with referring FIGS. 2 and 3. FIG. 2 is a sectional view of a transmission phase hologram or a diffraction optical element, which is made of the transparent light transmission resin 1a. A blaze 4, which is in response to phase difference, is formed in the transparent light transmission resin 1a in order to make phase difference occur to an incident light 2, and make transmission light generate the diffraction phenomenon and make the transmission light become a diffraction light 3a. The example shown in FIG. 2 is a 3 quantizing level phase hologram of which one phase difference is specified as 2.pi./3 by dividing one period, that is, 2 .pi. of the wavelength.lambda. of the incident light 2 by 3. The blaze 4 comprises 3 stages of phase difference including 0 .pi., which is a position of no phase shifting. A simplest phase hologram is a binary phase hologram of which one phase difference is specified as .pi. by dividing one period, that is, 2.pi. of the wavelength .lambda. of an incident light by 2. A blaze of a binary phase hologram comprises 2 stages of phase difference including 0.pi.. The more increases a number of values, the more increases diffraction efficiency. However, a production process becomes complicated.
FIG. 3 shows a general reflection phase hologram. In FIG. 3, a metallic reflection film 5 such as aluminum is formed on a blaze surface 1b in a transparent light transmission resin 1a and is coated with a protection film 6 in order to protect the metallic reflection film 5, and make the reflected light generate the diffraction phenomenon so as to make an incidental light 2 become a diffraction light 3b.
It seems, at a glance, hard to fake such a diffraction optical element mentioned above because the diffraction optical element can not be produced unless utilizing an expensive semiconductor manufacturing facility in conjunction with performing high-grade calculation by a computer. However, since the diffraction optical element is actually a simple blaze, there exists a problem that the blaze can be easily duplicated or faked by making a mold of the blaze with pouring liquid resin over the blaze and letting the resin harden, and by pouring liquid resin into the mold and letting the resin harden.
In the case of a transmission phase hologram shown in FIG. 2, for example, a peeling off layer can be formed on the blaze surface 1b of the blaze 4 by evaporating thin metal film on the blaze surface 1b of the blaze 4. A mold of which concave-convex shapes are formed inversely against the blaze 4 can be produced by pouring liquid resin over the peeling off layer of the blaze 4 and by peeling off the resin with the peeling off layer after the resin hardened. The peeling off layer of the mold is reinforced, and liquid resin is poured into the mold and hardened. Finally, a fake blaze can easily be produced.
In the case of a reflection phase hologram shown in FIG. 3, since the blaze surface 1b is covered with the metallic reflection film 5 and the protection film 6, it seems hard to fake a reflection phase hologram in comparison with a transmission phase hologram. However, the metallic reflection film 5 and the protection film 6 can easily be peeled off and then the blaze surface 1b can easily be exposed if they are soaked in the strong alkali solution. Accordingly, the blaze 4 can easily be faked as the same process as the faking process of the transmission phase hologram mentioned above.
In addition thereto, a height accuracy of the blaze surface 1b, which is faked or duplicated, deteriorates worse than that of the rue blaze surface 1b and then the diffraction efficiency deteriorates. Since the judging method of the card 1 whether it is a true card or a fake card depicted in FIG. 1 is based on the diffraction efficiency, it seems that such a fake blaze surface can easily be judged as fake. However, since the card 1 is directly handled by hand, stain such as a fingerprint, grease, dirt from hands or dust may stick on the surface of the card 1 and deteriorates the diffraction efficiency. Therefore, a threshold value of the diffraction efficiency utilized for judging whether it is true or fake is obliged to designate to rather lower value in consideration of such stain. Accordingly, there exists another problem that a fake produced as mentioned above can not be judged or discriminated properly.